1. Field of the Invention
This invention relates to the processing of semiconductor materials and, in particular, to the thinning or polishing of semiconductor materials.
2. Art Background
In the fabrication of semiconductor devices, it is often advantageous to form a device using processes such as metallization, epitaxial growth, and lithography on a relatively thick semiconductor substrate, i.e., a substrate with a thickness greater than 15 mils, and then later thin the substrate to its final thickness of a few mils. Additionally, during device fabrication, it is frequently necessary to polish a semiconductor surface before subsequent processing steps, e.g., epitaxial deposition, are performed. In either case, it is advantageous to leave a flat, uniform surface after processing.
Various methods have evolved for accomplishing either thinning or polishing of semiconductor materials. For example, the use of an abrasive material in conjunction with a grooved polishing plate has been utilized to thin various materials. In contrast, for polishing, chemical etchants such as bromine/methanol are used on soft polishing substrates, e.g., a chemically resistant felt or a leather type material, to produce useful, polished semiconductor surfaces.
Although these procedures are quite adequate for their intended purposes, there are shortcomings associated with each procedure. When an abrasive material is used in conjunction with a grooved plate, thinning is rapidly accomplished. However, the quality of the surface is not exemplary. Typically, surfaces obtained through abrasive treatment show scratch marks on the order of the size of the abrasive particles. Because of this limitation, abrasives usually are used where rapid material removal is desired and where a scrath-free surface quality is not essential. In contradistinction, the use of chemical etchants in conjunction with soft polishing plates produces highly polished scratch-free surfaces. (See, for example, Sullivan et al, Journal of the Electrochemical Society, 110, 585 [1963].) The etchant produces uniform removal while the soft polishing plate insures a scratch free surface. Surfaces showing no appreciable scratches and having flatnesses at the center of the polished surface on the order of 1.mu. are obtainable. The use of soft polishing substrates, such as Pellon cloth, (a proprietary felt type cloth product of Geoscience Instruments Corp. New York) to preclude scratching and, thus, to obtain high polishes also, unfortunately, results in the serious rounding of the polished semiconductor material at its extremities. Although the central portion of the polished material has an excellent surface, the rounded edges are not useable. This waste is undesirable when expensive semiconductor materials such as GaAs are employed. When the wafer has been previously processed using epitaxy, metallization, and other costly techniques, this waste is even more undesirable.